How do you change the default directory for saving/opening files?
You can't as Blender doesn't give us a way to do this. As gandalf3 says above in the comments it should use the last directory you accessed a file from in your current session. If you find yourself going to the same place several times, you can add a bookmark under Bookmarks or see your last visited folders under Recent in the sidebar. There should also be system bookmarks for your drives so no need to type if you can help it.